The present invention relates to video coders and, more particularly, to predictive video encoders.
FIG. 1 illustrates a simplified video coding system. The system includes an encoder 100 provided in communication with a decoder 200 over a channel 300. The encoder 100 receives original video data at an input. It generates coded video data from the original video data and outputs the coded video data to the channel 300. The channel 300 may be a communication link, such as those provided by telecommunications networks or computer networks, or may be a memory such as an optical, electric or magnetic storage device. The decoder 200 retrieves the coded video data from the channel 300 and, by inverting the coding process performed by the encoder 100, reconstructs the original video data therefrom. Depending upon the coding/decoding techniques used, the reconstructed video data may be either an exact replica or merely a close approximation of the original video data.
Coded video data consumes less bandwidth than uncoded video data. Encoders 100 employ coding techniques that exploit redundancy in the uncoded video data. A variety of coding techniques are known; they vary in terms of bandwidth conservation and computational complexity imposed upon the encoder and/or decoder.
One type of known video coder is the “predictive coder.” In a predictive coder, coded video data at one time may be coded using video data at another time as a reference. An example is shown in FIG. 2. FIG. 2 illustrates several frames 10–50 of video information, each frame representing the video data at different times. An intra-coded (I) frame 10, 50 is coded “from scratch.” That is, an I frame may be decoded based solely on the coded video data for that frame; the coded video data makes no reference to data of any other frame. By contrast, predictively coded (P) frames 20–40 are coded with reference to other frames. To decode a P frame (e.g, 20), a decoder 200 retrieves coded data for both the P frame and reconstructed data previously decoded for another frame (e.g. I frame 10). Prediction arrows 60–80 illustrate possible prediction directions. A frame usually occupies less bandwidth when coded as a P frame rather than as an I frame.
Predictive coders establish “prediction chains,” a series of frames wherein reconstructed data of one frame is used to predict the reconstructed data of another. Frames 10–40 in FIG. 2 illustrate a four frame prediction chain. An originating frame is coded as an I frame but all others in the chain are coded a P frames. Because the P frames achieve more bandwidth conservation than I frames, many coders extend prediction chains as far as possible. Some coding systems also force an encoder 100 to introduce an I frame even when unreasonable coding errors would not otherwise be present. By introducing I frames at regular intervals, decoders 200 may perform random access functions, akin to fast-forwarding and rewinding, that would not otherwise be possible.
As is known, predictive coders may code input video data on a block basis. Input video data may be broken down in to “blocks” (also called “macroblocks” in some coding applications), arrays of video data of a predetermined size. Each block of a frame is coded independently of the other blocks in the frame.
In a P frame, a predictive coder may code a block of data as one of three types:                (a) An Intra-Coded Block: The block is coded without reference to any block in any other frame;        (b) An Inter-Coded Block Sans Residual: Image data for the block is copied from a block in another frame and displayed without modification; or        (c) An Inter-Coded Block Plus a Residual: Image data for the block is copied from a block in another frame and supplemented with an error term (called a “residual”) that is supplied in the channel 300.        
FIG. 3 is a block diagram that illustrates processing that may be performed by an encoder 100 to code video data predictively. The encoder 100 may include a block encoder 110 and a block decoder 120. The block encoder 110 receives a block of input video data at a first input and blocks of decoded video data from the block decoder 120. For a P frame, the block encoder 110 determines how to code the block (whether as type (a), (b) or (c) above). The block encoder 110 outputs coded video data to the channel 300. In this regard, operation of the block encoder 110 is well-known.
The block decoder 120 receives coded video data from the block encoder 110 and decodes it. The decoded video data of a frame is stored at the block decoder 120 ready to be input back to the block encoder 110 to be used as a reference for predictive coding of later frames. In this regard, operation of the block decoder 120 is well-known.
The block decoder 120 permits the encoder 100 to “see” the reconstructed video data that the decoder 200 (FIG. 1) will obtain by decoding coded video data from the channel 300. The block decoder 120 permits the block encoder 110 to generate accurate residuals when coding blocks according to type (c) above. The block decoder 120 prevents prediction errors from compounding over a series of frames that would occur if type (c) blocks were used to predict other type (c) blocks in a prediction chain.
Reconstruction of coded data is a computationally expensive task. Provision of the block decoder 120 in an encoder 100 increases its complexity and expense. There is a need in the art for an encoder 100 that is less expensive and more simplified in implementation than known encoders.